The principal aim of this project is to determine the computations and central neural mechanisms for encoding and representing species-specific communication sounds in primates. The squirrel monkey was chosen because of its varied and highly stereotyped repertoire of calls. The experiment adheres to the following strategy: The monkey's calls are analyzed spectrographically to identify their constituent components. A computer is used to model call components and to manipulate these models in the frequency, amplitude and time domains. Modified calls and their components are resynthesized from their models and are used as auditory stimuli to test the responses of neurons in the medial geniculate nucleus (MGN) to frequency ratios and time intervals among call components. This is the first annual report since the project began on June 1, 1991. The principal findings are summarized below. 1. The MGN of the squirrel monkey contains neurons whose responses are facilitated by combinations of components from particular species-specific vocalizations. In this respect, the combination-sensitive neurons in the squirrel monkey resemble those described for lower vertebrates (bats, songbirds, and frogs). Thus, the discovery of combination-sensitive neurons in the squirrel monkey supports the hypothesis that these neurons are of general importance in processing species-specific communication sounds in vertebrates, including humans. 2. The combination-sensitive neurons in the MGN of the squirrel monkey are sensitive to time intervals (delays) between signal elements. Additional studies are needed to determine the extent to which these neurons are selective for particular vocalizations and whether they are sensitive to frequency ratio or some other spectral characteristic. The delay sensitivity may be a mechanism by which activity in populations of such neurons permit discrimination among acoustically related vocalizations. Similar acoustic discriminations are known to be important in phoneme identification by humans. Thus, we expect that physiological results obtained in the squirrel monkey may provide some insight on the neural mechanisms of human speech perception. 3. Preliminary results suggest that the combination-sensitive neurons are located in the dorsal division of the MGN. Additional studies using physiological in and anatomical tracing techniques are needed to determine the functional organization of this region. The expectation is that results from this line of study will lead to a greater understanding of the processing of complex acoustic signals of biological importance in the primate brain.